THE ANIMALS OF THE AMERICAS
North and South America comprise the only continuous land mass that reaches from the north to south polar regions, a distance of more than 14,500 km (9,000 mi). The combined area of the two continents is 41.4 million sq km (16 million sq mi), in which are found all terrestrial biomes.
The two continents have been joined for the past two or three million years. Earlier South America was an island, set apart from the northern land mass for at least 60 million years. This gave time for animal species unique to the continent to evolve. After the Isthmus of Panama emerged, there was an interchange of animals between North and South America, much as that experienced by Eurasia and America during the Ice Ages. One of the animals found in both Eurasia and America is the polar bear. Its habitat is along the entire Arctic coast. It has even been sighted hunting seals on ice floes hundreds of miles at sea. The polar bear's heavy coat insulates it from the icy water and air. Thick hair growing between its toes keep it from slipping on the ice. The thick, white pelt made the animal a prized trophy and reduced its population. The bear is now protected throughout its range.
The musk ox, resident of the far north, also has had to be protected from excessive hunting. At one time it came very close to extinction. A member of the cow family, the musk ox has adapted to the bitter cold by developing a heavy, shaggy coat consisting of two parts — a coarse outer covering of long guard hairs and a soft inner coat so dense that neither cold nor moisture can penetrate.
Musk oxen form a defensive ring when threatened. Adults stand along the perimeter, heads and horns pointing out, and the calves cluster together inside. This defensive posture works well against the ox's chief enemy, wolves, but is of little avail when high-powered rifles are the enemy.
Wolves prey on many species in the north — musk ox, caribou, moose, deer, hares, and even rodents. These carnivores are among the most maligned of all animals, victims of false myths and legends and systematic programs of extermination. They are accused of attacking humans and destroying entire herds of domestic animals. But their depredations of livestock are less severe than often claimed. And unprovoked attacks by healthy wolves in North America on humans are unknown. Those recorded from Europe's Middle Ages are thought to have been made by rabid animals or hybrids.
The world will be a far lonelier place if the last wolf dies. As biologist Ernest P. Walker wrote in his book, Mammals of the World, "The howl of the wolf and coyote, which to some people is of more enduring significance than superhighways and skyscrapers, should always remain a part of our heritage."
Some Arctic wolves remain snow white year round, an adoption to their environment. Three other predators of the far north— the snowy owl, Arctic fox, and weasel— are white at least part of the year.
The life cycle of the snowy owl demonstrates the close relationship which can exist between predator and prey. This owl hunts hares and lemmings. When these mammals are plentiful, female owls lay clutches of seven to ten eggs. When the food supply drops, only one to three eggs are laid.
Lemmings are among the most plentiful animals of the far north. These tiny rodents, found throughout the Arctic, are characterized by wide fluctuations in population. When vegetation is plentiful, the lemmings' numbers skyrocket. This population density seems to trigger a drive to migrate. Hordes of lemmings move out. Nothing deters them — swamps, forests, lakes, rivers. Eventually some reach the sea, which seems just one more obstacle. They plunge in, swim out, and drown.
Each summer the far north comes alive with the millions of birds which have migrated from the south to mate, build nests and raise their young. Waterfowl make up the majority of these migrants. Shore birds, pelagic birds, geese and ducks abound in the short Arctic summer. Some have come thousands of miles. The champion migrant is the Arctic tern, which flies • 16,000 km (10,000 mi) from the Antarctic, and in autumn flies back again.
When the birds leave the Arctic at the end of summer, they follow ancient flyways south. One of the flyways follows the Pacific coastline from Alaska to California. Small ponds and estuaries along the coast resound to the gabbling of hundreds of ducks.
Реферат опубликован: 21/12/2008